The present invention generally relates to a printhead for a thermal inkjet printer print cartridge and more particularly to a thermal inkjet cartridge printhead and associated interconnect and method for making the same which involves the integration of driver and multiplexing transistor circuitry with thin film technology and ink flow control to yield a printhead having improved print quality, print speed, and lower cost.
A substantial demand exists for printing system of high efficiency and resolution. To satisfy this demand, thermal inkjet print cartridges have been developed which print in a rapid and efficient manner. These cartridges include an ink reservoir in fluid communication with a multilayer printhead substrate having a plurality of resistors disposed in at least one of the layers. Selective electrical activation of the resistors causes a rapid boiling of the ink proximate to the activated resistors and expulsion of the ink from orifices in the printhead of the cartridge. Known representative thermal inkjet systems are discussed in U.S. Pat. Nos. 4,500,895; 4,514,298; and 4,794,409; the Hewlett-Packard Journal. Vol. 36, No. 5 (May 1985); and the Hewlett-Packard Journal, Vol. 39, No. 4 (August 1988).
In recent years, research has been conducted in order to increase the degree of print resolution, throughput, and quality of thermal inkjet printing systems. Print resolution depends on the number of ink-ejecting orifices and heating resistors formed on the cartridge printhead substrate. Modern circuit fabrication techniques allow the placement of substantial numbers of resistors on a single printhead substrate. However, the number of resistors applied to the substrate is limited by the conductive components used to electrically connect the cartridge to external driver circuitry in the printer unit. Specifically, an increasingly large number of resistors requires a correspondingly large number of interconnection pads, leads, and the like. This increase in components and interconnect causes greater manufacturing/production costs, and increases the probability that defects will occur during the manufacturing process.
In order to solve this problem, thermal inkjet printheads have been developed which incorporate pulse driver circuitry (e.g. metal oxide semiconductor field effect (MOSFET) transistors) directly on the printhead substrate with the resistors. This development is described in U.S. Pat. Nos. 4,719,477; 4,532,530; and 4,947,192. The incorporation of driver circuitry on the printhead substrate in this manner reduces the number of interconnect components needed to electrically connect the cartridge to the printer unit. This results in an improved degree of production and operating efficiency.
To produce high-efficiency, integrated printing systems as described above, significant research has been conducted in order to develop improved MOSFET transistor structures and methods for integrating the same into thermal inkjet printing units. Currently, MOSFET devices are manufactured using a substantial number of conventional masking/etching steps. However, it is always desirable in the production of MOSFET devices and thermal inkjet printing systems to reduce the number of necessary materials and manufacturing steps. This results in lower production costs and greater manufacturing efficiency. An integration of driver components and printing resistors onto a common substrate would result in a need for specialized, multi-layer connective circuitry so that the driver transistors can communicate with the resistors and other portions of the printing system. Typically, this connective circuitry involves a plurality of separate conductive layers, each being formed using conventional circuit fabrication techniques. However, this procedure again results in increased production costs and diminished manufacturing efficiency.
To create the resistors, conventionally, an electrically conducting layer is positioned on selected portions of the layer of resistive material in order to form covered sections of the resistive material and uncovered sections thereof. The uncovered sections ultimately function as heating resistors in the printhead. The covered sections are used to form continuous conductive links between the electrical contact regions of the transistors and other components in the printing system (e.g. the heating resistors). Thus, the layer of resistive material performs dual functions: as heating resistors in the system, and as direct conductive pathways to the drive transistors. This substantially eliminates the need to use multiple layers for carrying out these functions alone.
A selected portion of protective material is then applied to the covered and uncovered sections of resistive material. Thereafter, an orifice plate having a plurality of openings through the plate is positioned on the protective material. Beneath the openings, a section of the protective material which was removed forms ink firing cavities or chambers. Positioned at the bottom surface of each chamber is one of the heater resistors. The electrical activation of each resistor causes the resistor to rapidly heat and vaporize a portion of the ink in the cavity. The rapidly formed (nucleated) ink bubble ejects a droplet of ink from the orifice associated with the activated resistor and ink firing cavity.
Once the heater resistors have been placed closer together, the orifices (printhead nozzles) must also be placed more closely together to realize higher quality print. By placing nozzles closer together, the print quality can be improved. By placing more nozzles on the print head, the width of the printing swath is increased. However, adding resistors and nozzles requires adding associated power and control interconnections. These interconnections are conventionally flexible wires or equivalent conductors that electrically connect the transistor drivers on the printhead to printhead interface circuitry in the printer. They may be contained in a ribbon cable that connects on one end to control circuitry within the printer and on the other end to driver circuitry on the printhead. More heater resistors spaced closer together also creates a greater likelihood of crosstalk and increased difficulty in supplying ink to each firing chamber quickly.
Interconnections are a major source of cost in printer design, and adding them to increase the number of heater resistors increases the cost and reduces the reliability of the printer. Thus, as the number of drivers on a printhead has increased over the years, there have been attempts to reduce the number of interconnections per driver. A matrix approach offers an improvement over the direct drive approach, yet as previously realized a matrix approach has its drawbacks. The number of interconnections with a simple matrix is still large and still results in an undesirable increase in the number of interconnections
Another concern with inkjet printing is the sufficiency of ink flow to the paper or other print media. Print quality is also a function of ink flow through the printhead. Too little ink on the paper or other media to be printed upon produces faded and hard-to-read printed documents. In a worst case, no ink may be printed and the entire document is lost. This scenario may occur where a facsimile machine, out of ink, receives a transmission when unattended and attempts to print. Since the inkjet pen moves across the media even when no ink is being ejected, the facsimile machine mistakenly assumes that the transmission has successfully been received and acknowledges reception to the sender.
Ink flow from its storage space to the ink firing chamber has suffered, in previous printhead designs, from an inability to be rapidly supplied to the firing chambers. The manifold from the ink source inherently provides some restriction on ink flow to the firing chambers thereby reducing the speed of printhead operation as well as resulting in crosstalk.
To resolve these needs of increased printing speed and quality, reduced number of interconnections, and improved ink flow control, a modem design of thermal ink jet printer printhead is desirable.